The present invention relates to scaly silica particles having a self film-forming property and being capable of forming a strong silica coating film even at normal temperature. More particularly, it relates to scaly silica particles capable of forming a strong silica coating film having acid resistance, alkali resistance and heat resistance at the same time, or a hardenable composition or an adhesive composition containing them, and a process for their production.
This hardenable composition is useful for various applications, such as a particle binder, a coating material or agent for exterior or interior of buildings or structures, a coating material or agent having a thermal function (such as heat resistance, heat insulation, fire proofing, or flame retardancy), a coating material or agent having an optical function (such as ultraviolet ray-shielding, selective adsorption of light, or light emission/fluorescence), a coating material or agent having an electrical/magnetical function (such as electrical insulation, electrical conductivity, antistatic function, wave-absorbing or electromagnetic wave-shielding), a coating material or agent having an adsorbing function (such as adsorption and desorption of moisture, adsorption and desorption of gas, or thin layer chromatography), a particle binder for adsorbent particles, a coating material or agent having a catalytic function (such as a photo-oxidation catalyst), a particle binder for catalyst particles, a coating material or agent for a biological function (such as antibacterial, antiseptic, ship bottom anti-fouling, aquaculture or cell culture) and a coating material or agent having a fragrance or deodorant function.
Further, the present invention relates to an adhesive composition capable of forming a strongly bonded product even at a normal temperature. More particularly, it relates to an adhesive composition capable of forming strongly bonded hardened layers having acid resistance, alkali resistance and heat resistance at the same time.
In the present invention, the substrate to which the hardenable composition, specifically, the adhesive composition or the coating material or agent, is applied, may, for example, be a metal, glass, ceramics, a cement hardened product, a plastic, wood or paper, and the form of the substrate may be any one of a flat plate form, a curved surface form, a porous form, a fibrous form, a woven fabric form, a non-woven fabric form or a net structure form and is not particularly limited to any specific form.
Heretofore, for organic coating materials, various materials have been used including highly durable coating materials such as fluorine resin coating materials or acrylic silicon resin coating materials. However, on the other hand, with such organic coating materials, it is necessary to take into consideration various problems specific to them, such as air pollution due to a volatile organic compound (VOC) in a room, provision of an anti-fouling property, provision of moisture permeability or respiratory property, and elimination of an organic solvent.
Further, when photo-oxidation catalyst particles are employed for antibacterial or anti-fouling purposes, an organic coating material or binder is used to fix the particles on a substrate, but such is usually poor in the durability and has many problems such as fire proofing or flame retarding problems. Accordingly, a study has been directed to develop an inorganic coating material or agent which does not essentially have such problems.
As an inorganic coating material or agent, or a particle binder, a silicon type material has been used in many cases from the viewpoint of the safety to human bodies and environment. Among silicon type materials, as a material having a coating film-forming function or a particle-binding function, {circle around (1)} an alkali metal silicate (water glass type) aqueous solution, {circle around (2)} colloidal silica, {circle around (3)} an organo alkoxy silane and {circle around (4)} bentonite or smectite, have, for example, been known.
However, {circle around (1)} the alkali metal silicate aqueous solution is highly alkaline and has a problem in the safety in operation, and further, it requires hot air treatment or heating treatment for hardening, whereby hardening at normal temperature is difficult. {circle around (2)} colloidal silica is suitable for a particle binder or a coating agent and is widely employed, but the thickness of the coating which can be formed by colloidal silica alone is usually limited to less than 10 xcexcm, normally less than 5 xcexcm, and if the thickness of the coating is thicker, the coating tends to have cracks, whereby no strong coating film can be obtained. Besides, usually, heat treatment is required to improve the strength of the coating film. Further, colloidal silica is amorphous silica, and as such, it has acid resistance, but has a drawback that it is poor in alkali resistance.
On the other hand, {circle around (3)} the organo alkoxy silane has a drawback such that it requires heat treatment for hardening, or it is poor in alkali resistance. Further, it is essentially an organic material, and the obtainable hardened body does not have flame retardancy. Further, {circle around (4)} each of bentonite and smectite is poor in the water resistance, acid resistance and alkali resistance.
Further, among silicon type materials, silicas are also known. However, silicas other than colloidal silica, such as silica gel, hydrous silicic acid (so-called white carbon), quartz, cristobalite and tridymite, have problems that they are very poor in the film-forming function (self film-forming property) without a particle binder.
In any case, as mentioned above, among conventional silica type coating film-forming materials, there has been none which has all of a normal temperature hardenable property, a thick-coating property and a self film-forming property (coating properties and binding properties) and which is capable of forming a silica coating film having all of acid resistance, alkali resistance and heat resistance.
On the other hand, as an adhesive composition, various organic adhesives and inorganic adhesives have, heretofore been used. The organic adhesives have a merit that they can be hardened at normal temperature but have a drawback that they are poor in the organic solvent resistance or heat resistance.
Whereas, with the inorganic adhesives, the organic solvent resistance is usually high, and the heat resistance is also usually high, but for their hardening, each of them requires heat treatment at a temperature of from 100 to 150xc2x0 C., such being inconvenient.
Accordingly, a study has been directed to develop an adhesive which can be hardened at room temperature and which has heat resistance, acid resistance, alkali resistance and organic solvent resistance at the same time.
Heretofore, as an adhesive composition containing a silica component and having an adhesive function, colloidal silica or an aqueous alkali metal silicate solution has been known. However, the colloidal silica has a problem such that the alkali resistance is poor, and heat treatment at a temperature of from 400 to 600xc2x0 C. is required for its hardening. Further, an aqueous adhesive containing such colloidal silica can hardly form a thick adhesive layer and can hardly provide a stabilized adhesive force.
On the other hand, the alkali metal silicate has a problem that heat treatment at a temperature of from 100 to 600xc2x0 C. is required for its hardening, and a thick adhesive layer can likewise hardly be formed.
Further, with an aqueous solution of a water-soluble polymer such as polyvinyl alcohol, polyvinyl pyrrolidone, methyl cellulose, carboxy methyl cellulose or gelatin, it is possible to form an adhesive layer, but when such an aqueous solution is used alone, there will be a problem in the durability of the adhesive layer.
Further, other silicas, i.e. silicas other than colloidal silica, such as silica gel, hydrous silicic acid (so-called white carbon), quartz, cristobalite and tridymite, do not have adhesive functions.
In any case, as mentioned above, among the conventional adhesive compositions, there has been none which has all of a normal temperature hardenable property, organic solvent resistance, acid resistance, alkali resistance and heat resistance.
It is an object of the present invention to provide fine scaly silica having such drawbacks of the conventional silicon type materials basically overcome, which may be used alone, a hardenable composition or an adhesive composition containing it, a hardened product thereof, and a process for producing it.
The present inventors have conducted an extensive study in consideration of the importance of the above object in the field of coating materials or agents, and as a result, have found that in the synthesis of fine scaly low crystalline silica, by controlling the microstructure of the particle form, it is possible to obtain fine scaly silica having a laminated structure, consisting essentially of secondary particles of foliar silica formed by a plurality of flaky primary particles which are overlaid one on another and aligned face-to-face in parallel with one another, said secondary particles being present independent of one another, which has a high self film-forming property, and a composition containing such fine scaly silica can form a coating material, a coating agent or an adhesive having excellent functions. The present invention has been accomplished on the basis of these discoveries.
Namely, the present invention provides the following:
Scaly Silica
(1) Scaly silica particles having a laminated structure, consisting essentially of secondary particles of foliar silica, each formed by a plurality of flaky primary particles of scaly silica which are overlaid one on another and aligned face-to-face in parallel with one another, said secondary particles being present independent of one another.
Hardenable Composition or Adhesive Composition
(2) A hardenable composition or an adhesive composition comprising the scaly silica particles having the laminated structure as defined in (1) and a volatile liquid.
Here, the volatile liquid means water and/or a volatile liquid other than water (such as a volatile organic solvent).
Hardenable Composition for Forming Heat Insulating Coating
(3) A hardenable composition for forming a heat insulating coating, comprising the scaly silica particles having the laminated structure as defined in (1), a volatile liquid and a material having a heat-insulating function.
Hardened Product or Hardened Coating Film
(4) A hardened product or hardened coating film, comprising secondary particles of foliar silica formed by a plurality of flaky primary particles of scaly silica overlaid one on another and aligned face-to-face in parallel with one another, wherein the secondary particles are laminated in parallel with one another.
Process for Producing Scaly Silica Particles
(5) A process for producing scaly silica particles suitable for a hardenable composition, which consist essentially of secondary particles of foliar silica and have a laminated structure wherein the secondary particles are present independent of one another, said process comprising:
(i) a step of subjecting either silica hydrogel, silica sol or hydrous silicic acid to hydrothermal treatment in the presence of an alkali metal salt to form tertiary agglomerated particles of scaly silica comprising secondary particles of foliar silica, each formed by a plurality of flaky primary particles of scaly silica which are overlaid one on another and aligned face-to-face in parallel with one another, and tertiary particles formed by the secondary particles which are overlaid one on another three dimensionally irregularly to have pore spaces, and
(ii) a step of disintegrating and dispersing the tertiary agglomerated particles of scaly silica to form foliar silica particles consisting substantially of secondary particles containing substantially no tertiary particles.